Hydroprocessing can include processes which convert hydrocarbons in the presence of hydroprocessing catalyst and hydrogen to more valuable products.
Hydrocracking is a hydroprocessing process in which hydrocarbons crack in the presence of hydrogen and hydrocracking catalyst to lower molecular weight hydrocarbons. Depending on the feed characteristics and desired products, a hydrocracking unit may contain one or more beds of the same or different catalyst.
Due to environmental concerns and newly enacted rules and regulations, saleable fuels must meet lower and lower limits on contaminates, such as sulfur and nitrogen. New regulations require essentially complete removal of sulfur from diesel. For example, the ultra low sulfur diesel (ULSD) requirement is typically less than about 10 wppm sulfur.
Hydrotreating is a hydroprocessing process used to remove heteroatoms such as sulfur and nitrogen from hydrocarbon streams to meet fuel specifications and to saturate olefinic compounds. Hydrotreating can be performed at high or low pressures, but is typically operated at lower pressure than hydrocracking.
Hydroprocessing recovery units typically include a stripper for stripping hydroprocessed effluent with a stripping medium, such as steam, to remove unwanted hydrogen sulfide. The stripped effluent then is heated in a fired heater to fractionation temperature before entering a product fractionation column to recover products such as naphtha, kerosene and diesel.
Hydroprocessing, and particularly hydrocracking, is very energy-intensive due to the severe process conditions such as the high temperature and pressure used. Over time, although much effort has been spent on improving energy performance for hydrocracking, the focus has been on reducing reactor heater duty. However, a large heater duty is required to heat stripped effluent before entering the product fractionation column.
The traditional hydroprocessing design features one stripper which receives two feeds, a relatively cold hydroprocessed effluent stream which may be from a cold flash drum and a relatively hot hydroprocessed effluent stream which may be from a hot flash drum. Although these two feeds contain very different compositions, they can be traced back to the same location from a hydroprocessing reactor and perhaps, a hot separator. An overhead vapor stream of the hot separator may go to a cold separator and the liquid from the cold separator may go to a cold flash drum while a bottoms liquid of the hot separator may go to a hot flash drum. Traditionally, the liquid of both hot and cold flash drums are fed to a single stripper. A stripper bottoms stream may become the feed for the product fractionation column. The inefficiency of this one-stripper design is rooted in mixing of the liquids of the hot flash drum and the cold flash drum in the same stripper which partially undoes the separation previously accomplished in the hot separator.
A conventional hydroprocessing process using a single stripping column is illustrated in FIG. 1. Feed 5 mixes with hydrogen rich recycle gas, is heated, and enters the hydroprocessing reactor 10. The effluent 15 from the reactor 10 is partially cooled and sent to a hot separator 20 where it is separated into a vapor stream 25 and a liquid stream 30. The hot separator 20 operates at elevated temperature, typically about 288° C. (550° F.) to about 315° C. (600° F.).
The liquid stream 30 from the hot separator 20 is sent to a hot flash drum 35 which operates at a lower pressure, typically about 1724 kPa (g) (250 psig) to about 2758 kPa (g) (400 psig). The liquid 40 from the hot flash drum 35 is sent to the stripper column 50.
The vapor stream 25 from the hot separator 20 contains the recycle gas and some vaporized product range hydrocarbons. The vapor stream 25 is cooled to between 60° C. (140° F.) and 43° C. (110° F.) and sent to the cold separator 55. The gas stream 56 from the cold separator may be scrubbed in the recycle gas scrubber 130 which uses an amine to remove hydrogen sulfide. The scrubbed recycle gas 57 is compressed by the recycle gas compressor 135, mixed with makeup gas 150 and the resulting recycle gas 59 is mixed with fresh feed.
The liquid stream 60 from the cold separator 55 is let down in pressure and sent to the cold flash drum 65, which operates at a pressure between about 1724 kPa (g) (250 psig) to about 2758 kPa (g) (400 psig). Gases 70 evolved when the cold separator pressure is let down are separated.
The cold flash liquid 75 is heated to a temperature required by heat balance in order to achieve the objectives of the stripper 50. The heating may be accomplished by exchange with available heat elsewhere in the unit 80.
The bottoms stream 85 from the stripper column 50 may be further heated in exchanger 86 and sent to a separator 90. The vapor stream 95 from the separator 90 is sent to fractionator 100. The liquid stream 105 from the separator is heated in fired heater 110 and then sent to product fractionator 100. The product fractionator separates the vapor and liquid into various hydrocarbon product streams 105, 115, 120, and 125. To improve the separation between the bottoms product 125 and the first sidedraw stream 120, a stripping medium, such as steam, 126 is added.
However, conventional units with single stripper columns are expensive to operate because of the heating required for the streams entering the fractionator.
Therefore, there is a need for more energy efficient methods of separating and recovering the products produced in the reactor.